Of filters

ABSTRACT

An electric filter comprises a plurality of thin film bulk acoustic resonants ( 10, 11 ) arranged in series and in parallel which have electrodes ( 24, 25 ) of different working area (L) and optionally a piezoelectric layer ( 23 ) of different thickness (T).

[0001] The present invention relates to a filter comprising plural thinfilm bulk acoustic resonators (FBARs) and more particularly, is directedto a piezoelectric filter which is manufactured using thin filmtechnology in order to obtain a low close-in rejection and a lowout-of-band rejection at high frequency.

[0002] Thin film bulk acoustic resonators (FBARs) are attractive devicessince they show resonant peaks at high frequency, particularly in theMHz and GHz regions. Moreover, FBARs can be achieved in a small devicesize (˜mm). Thus they are considered to be useful not only asresonators, but to be applied for filters, transducers, etc., whichwould be installed in small, light, thin electric appliance products,such as a mobile phone. It is known that an electric filter can be madewith plural FBARs. That kind of filter can work in MHz frequency regionor in GHz frequency region. Generally speaking, for a band-pass filter,the rejection level compared to signal-pass area improves when moreFBARs are used for the filter.

[0003]FIG. 9 shows one example of a filter comprising four FBARs. Thefour FBARs are separated into 2 groups according to their functions inthe filter. FBAR1 and FBAR2 in FIG. 9 are connected in series. Thereforethey are in one group. Also, FBAR3 and FBAR4 are connected in paralleland in the other group. Usually those FBARs are prepared on onesubstrate and prepared under the same procedure. Therefore each FBARconsists of exactly the same structure. In other words, the samethickness and the same materials for all layers on each FBAR. And also,usually each FBAR has the same working area size.

[0004] It is important to prepare a band-pass filter at high frequencyin the MHz or GHz region because those frequency regions are often usedfor wireless communications these days. For a band-pass filter, it isextremely important that both the close-in rejection and the out-of-bandrejection are low. Therefore, a technique has been demanded to make afilter with a good close-in and out-of-band rejection.

[0005] An object of the present invention is to prepare a filter,showing a low close-in rejection and a low out-of-band rejection whichcomprises plural FBARs. By this invention, these properties can beobtained by changing the sizes of the working areas between FBARs inseries and in parallel.

[0006] The filter comprises a plurality of FBARs in which at least oneFBAR is in series and one FBAR in parallel.

[0007] Each FBAR comprises a plurality of layers consisting of (fromlower to upper): a substrate, a dielectric layer, one or more metallayers acting as a lower electrode, a piezoelectric layer, one or moremetal layers acting as an upper electrode.

[0008] An FBAR exhibits a series resonance and a parallel resonance atrespective frequencies that are functions of the thicknesses of all ofthe layers. In such a filter, it is useful to alter the sizes of theworking areas between the FBARs in series and the FBARs in parallel.

[0009] The working area is usually restricted with the area sizes ofelectrodes in contact with the piezoelectric layer to form the FBARs. Inthis case, the resonant peaks are exhibited as different shapes of thereactance curves for FBARs in series relative to FBAR's in parallel.

[0010] As a result of making a filter using FBARs in series and inparallel with different working areas, a filter which shows a lowclose-in rejection and a low out-of-band rejection can be obtained.

[0011] The theory is described using FIG. 9. In FIG. 9, if all FBARs areprepared under the same condition, both FBARs in series (FBARs 1 and 2)and FBARs in parallel (FBARs 3 and 4) exhibit exactly the same resonantpeaks. Then, if the thickness of the piezoelectric layer is changed forFBARs in series relative to FBARs in parallel, the resonant peaks appearat a different frequency but the shape of the resonant peaks is thesame, as shown in FIG. 10(a). As a result of making a filter using thoseFBARs which comprise a different thickness of the piezoelectric layerfor FBARs in series relative to FBARs in parallel, a through signal(S₂₁) of the filter is obtained, as shown in FIG. 10(b). In FIG. 10(b),close-in rejections appear at frequencies of f_(s2) and f_(p1) and anout-of-band rejection is at a frequency region below f_(s2) and abovef_(p1).

[0012] On the other hand, if in FIG. 9, it is assumed that the size ofthe top electrode and bottom electrode on FBARs in series (FBARs 1 and2) and on FBARs in parallel (FBARs 3 and 4) are different but the otherdimensions on all FBARs are supposed to be the same as the filter inFIG. 10, then FBARs in series and FBARs in parallel show differentshapes of reactance curves, as shown in FIG. 11(a). In FIG. 11(a), theshapes of the curves are different because of the differences betweenthe capacitances of the working areas. As a result of preparing a filterusing those different FBARs in series and in parallel, a filter whichshows a low close-in rejection, particularly at f_(s2), and a lowout-of-band rejection can be obtained, as shown in FIG. 11(b).

[0013] In order to alter the sizes of electrodes, one example would beto change the mask designs for the electrodes for the FBARs in seriesrelative to the FBARs in parallel.

[0014] The invention will now be described in more detail, by way ofexample only, with reference to the accompanying drawings, wherein:

[0015]FIG. 1 illustrates a schematic diagram of a preferred filter whichcomprises three FBARs in series and three FBARs in parallel.

[0016]FIG. 2 illustrates a top view and a cross section view of a FBAR.

[0017]FIG. 3 illustrates the S parameters of a FBAR which compriseselectrodes whose size is 110 microns square and a piezoelectric layerwhose thickness is 1.2 microns, and which is used as FBARs in series ona preferred filter.

[0018]FIG. 4 illustrates S parameters of a FBAR which compriseselectrodes whose size is 285 microns square and a piezoelectric layerwhose thickness is 1.255 microns, and which is used as FBARs in parallelon a preferred filter.

[0019]FIG. 5 illustrates a S₂₁ curve of a preferred filter whichcomprises three similar FBARs in series, comprising electrodes whosesize is 110 microns square and a piezoelectric layer whose thickness is1.2 microns, and which comprises three similar FBARs in parallel,comprising electrodes whose size is 285 microns square and apiezoelectric layer whose thickness is 1.255 microns.

[0020]FIG. 6 illustrates the S parameters of a FBAR which compriseselectrodes whose size is 110 microns square and a piezoelectric layerwhose thickness is 1.2 microns, and which is used as FBARs in series ona compared filter.

[0021]FIG. 7 illustrates S parameters of a FBAR which compriseselectrodes whose size is 110 microns square and a piezoelectric layerwhose thickness is 1.255 microns, and which is used as FBARs in parallelon a compared filter.

[0022]FIG. 8 illustrates a S₂₁ curve of a compared filter whichcomprises three similar FBARs in series, comprising electrodes whosesize is 110 microns square and a piezoelectric layer whose thickness is1.2 microns, and which comprises three same FBARs in parallel,comprising electrodes whose size is 110 microns square and apiezoelectric layer whose thickness is 1.255 microns.

[0023]FIG. 9 illustrates a schematic diagram of electrical equivalentcircuit for a filter which comprises two FBARs in series and two FBARsin parallel.

[0024]FIG. 10(a) illustrates the resonant curves of FBARs whichcomprises different thickness of a piezoelectric layer for FBARs inseries relative to FBARs in parallel.

[0025]FIG. 10(b) illustrates a S₂₁ curve of a filter which comprises twotypes of FBARs, comprising different thickness of a piezoelectric layer,for two FBARs in series relative to two FBARs in parallel.

[0026]FIG. 11(a) illustrates the resonant curves of FBARs whichcomprises different thickness of a piezoelectric layer and differentsize of electrodes for FBARs in series relative to FBARs in parallel.

[0027]FIG. 11(b) illustrates a S₂₁ curve of a filter which comprises twotypes of FBARs, comprising different thickness of a piezoelectric layerand different size of electrodes, for two FBARs in series relative totwo FBARs in parallel.

[0028] According to a preferred embodiment of the invention, the filteris prepared with FBARs in series and in parallel which have differentsizes of electrodes and different thicknesses of piezoelectric layer.The embodiment may be understood in view of FIGS. 1 and 2, whichillustrate respectively a schematic diagram of the filter and across-section of each FBAR in the filter.

[0029] The three FBARs in series (10) are indicated in FIG. 1 as are thethree FBARs in parallel (11). Those six FBARs are prepared on onesubstrate. The FBARs in series 10 and in parallel 11 have the structureshown in FIG. 2. The FBARs in series 10 and in parallel 11 comprise atop electrode 24, a bottom electrode 25, a piezoelectric layer 23 and abridge or membrane layer 22 on a Si wafer 20 which is etchedanisotropically using a backside pattern layer 21. There are two pointsof difference between the FBARs in series 10 and in parallel 11. One isthe thickness of the piezoelectric layer 23 (shown as T in FIG. 2), andthe other is the size of the top electrode 24 and the bottom electrode25 (shown as L in FIG. 2).

[0030] The preparation procedure of the filter, comprising six FBARs, isdescribed as follows. At first, silicon nitride (SiN_(x)) is coated at200 nm with chemical vapour deposition on both sides of a bare Si wafer20. The SiN_(x) on the front side of the Si wafer 20 is a membrane layer22. A backside pattern layer 21 is prepared on the back side of the Siwafer 20 in the SiN_(x) with photolithography and reactive ion etching.A bottom electrode 25 is prepared with so-called a lift-off processwhich is carried out as follows. First a pattern of photoresist isprepared with photolithography. Then, chromium and gold (Cr/Au) aredeposited by sputtering at thicknesses of 10 nm and 100 nm respectively.Cr is used as an adhesion layer. Next, the patterned photoresist andCr/Au on it is removed with acetone, because the photoresist dissolvesin acetone. After that procedure, a bottom electrode 25 is obtained.Next, Zinc Oxide (ZnO) is deposited for a piezoelectric layer 23 bysputtering. The thickness of the piezoelectric layer 23 is 1.2 micronsfor FBARs in series 10 and 1.255 microns for FBARs in parallel 11,respectively. In order to obtain a piezoelectric layer 23 which shows adifferent thickness, a photolithography and an etching are employed. Thepiezoelectric layer 23 is etched with acetic acid to make a contact hole26 in order to touch a bottom electrode 25 with an electrical probe.Afterwards, a top electrode 24 is prepared by the lift-off process. TheCr and Au thickness of a top electrode 24 is set at 10 nm and 100 nmrespectively. The top electrode 24 has a transmission line and a squareworking area on which one dimension shown as L in FIG. 2. The workingarea size is the same for the bottom electrode 25. When the topelectrode 24 is prepared, two ground electrodes 27 are prepared as wellunder the same lift-off process, so the top electrode 24 has a coplanarwave-guide structure for which the characteristic impedance is set atabout 50 ohms.

[0031] The size of the working area, which is equal to the centre partof the top electrode 24 and bottom electrode 25, is 110 microns squarefor FBARs in series 10 and 285 microns square for FBARs in parallel 11.The mask design for both electrodes is changed between FBARs in series10 and FBARs in parallel 11.

[0032] Finally, the Si wafer 20 is etched from its backside with KOHsolution, using the backside pattern layer 21 and the preparationprocess for the filter is finished.

[0033] A network analyser is used for the electrical measurement. Atfirst, a measurement is carried out on each FBAR in the filter. EachFBAR, which is not connected to other FBARs, is prepared individually inorder to measure the electrical response separately.

[0034]FIG. 3 shows the S parameters of an FBAR in series 10, whichcomprises a piezoelectric layer 23 at 1.2 microns thick and a workingarea at L=110 microns. S₁₁ is the reflection coefficient at the RF powersource and S₂₁ is the transmission coefficient measured at the outputport. Series and parallel resonant peaks on S₁₁ and S₂₁ are presented atthe frequency of 1.585 GHz and 1.636 GHz, respectively.

[0035] On the other hand, FIG. 4 shows the S parameters of a FBAR inparallel 11, which comprises a piezoelectric layer 23 at 1.255 micronsthick and a working area at L=285 microns. All the resonant peaks in S₁₁and S₂₁ are shifted to a lower frequency relative to the same kind ofpeaks in FIG. 3. The series resonant peak on S₁₁ in FIG. 4 appears at1.543 GHz and the parallel resonant peak on S₁₁ in FIG. 4 and a resonantpeak on S₂₁ in FIG. 4 are presented at 1.585 GHz which is the samefrequency as for the series resonant peak in FIG. 3. Also, the shapes ofS₁₁, and S₂₁ in FIG. 4 are different from those in FIG. 3 due to thedifference of the capacitance of a working area of FBARs.

[0036] Using those two kinds of FBARs, the filter is fabricated usingthe configuration in FIG. 1. The through coefficient (S₂₁) of the filteris shown in FIG. 5. Close-in rejections of the filter appear at 1.540GHz and at 1.635 GHz. One close-in rejection at 1.540 GHz is due to aseries resonant peak of S₁₁ on FBARs in parallel 11 at 1.543 GHz in FIG.4. The other close-in rejection at 1.635 GHz is due to a parallelresonant peak of S₂₁ on FBARs in series 10 at 1.636 GHz in FIG. 3. Anout-of-band rejection in a frequency region below 1.540 Ghz and above1.635 GHz is less than −50 dB.

[0037] To compare with the preferred filter above, a filter fabricatedusing the same configuration in FIG. 1 is made. On the compared filter,FBARs for both FBARs in series 10 and FBARs in parallel 11 comprise aworking area at L=110 microns. The thickness of the piezoelectric layer23 is 1.2 microns for FBARs in series 10 and 1.255 microns for the FBARsin parallel 11.

[0038] At first, a measurement is carried out on each FBAR in thecompared filter. Each FBAR, which is not connected to the other FBARs,is prepared individually in order to measure the electrical responseseparately.

[0039]FIG. 6 shows S parameters of an FBAR in series 10, which comprisesa piezoelectric layer 23 at 1.2 microns thick and a working area atL=110 microns. Since the FBAR in series 10 in the compared filter is thesame as a FBAR in series 10 in the preferred filter, the S parameters inFIG. 6 are the same as those in FIG. 3.

[0040] On the other hand, FIG. 7 shows the S parameters of an FBAR inparallel 11 in the compared filter, which comprises a piezoelectriclayer 23 at 1.255 microns thick and a working area at L=110 microns. Allresonant peaks on S₂₁ and S₂₁ in FIG. 7 are shifted to lower frequencythan the same kind of peaks in FIG. 6. The series resonant peaks on S₁₁and S₂₁ in FIG. 7 appear at 1.543 GHz. The parallel resonant peaks onS₁₁ and S₂₁ are presented at 1.585 GHz which is the same frequency asfor the series resonant peak in FIG. 6. However, the shapes of S₁₁ andS₂₁ in FIG. 7 are the same as those in FIG. 6 because the working areais the same for both FBARs in series 10 and FBARs in parallel 11.

[0041] Using those two kinds of FBARs, the compared filter is fabricatedusing the configuration in FIG. 1. The through coefficient (S₂₁) of thecompared filter is shown in FIG. 8. Close-in rejections of the comparedfilter appear at 1.540 GHz and at 1.635 GHz. Those frequencies are thesame as in FIG. 5. Comparing the peak depth of one close-in rejection at1.540 GHz, a close-in rejection in FIG. 5 is deeper than that in FIG. 7.Moreover, comparing an out-of-band rejection, the preferred filter showsless out-of-band rejection than the compared filter on which anout-of-band rejection is around −22 dB in FIG. 8. The superiorperformance of the preferred filter is obtained because of a differentshape of S parameters between FBARs in series 10 and FBARs in parallel11. The different shapes of the S parameters are due to the differencein the capacitances of the FBAR's due to their different working areas.

[0042] Comparing the preferred filter with the compared filter, thepreferred filter shows a lower close-in rejection and a lowerout-of-band response.

[0043] The preferred filter described above is one example for thisinvention. However, the thin film techniques and materials for eachlayer on the preferred filter described above are not restricted tothose described. For example, the material for the piezoelectric layer23 is not restricted to ZnO. Aluminium nitride (AlN) which shows a highQ value and lead titanate zirconate (PZT) which shows a largeelectromechanical coefficient are also usable. Also, lead scandiumtantalum oxide and bismuth sodium titanium oxide are other examples ofpiezoelectric materials. The material for the top electrode 24 and abottom electrode 25 is not restricted with Cr/Au. Aluminium (Al) andplatinum (Pt), which are often used for electrodes, are also usable. Thematerial for the membrane layer 22 and a backside pattern layer 21 isnot restricted to SiN_(x). SiO₂ is also possible.

[0044] The numbers of FBARs in series 10 and FBARs in parallel 11 arenot restricted to 3 each. The numbers of FEBARs in series 10 and FBARsin parallel 11 should be decided by the specifications for the level ofclose-in rejection, area size for the filter and so on.

[0045] The FBARs which are used as FBARs in series 10 and FBARs inparallel 11, are not restricted to an FBAR which comprises an etchedhole on Si wafer 20 at the backside of a bottom electrode 25. Any airgap or a Bragg reflector may be used at the backside of the bottomelectrode 25. Therefore Si wafer 20 is not necessarily used as asubstrate for the FBARs.

[0046] It is sometimes usual to make electrodes larger than the workingarea to make a contact area of measurement probes, for instance. In thiscase, the working area is restricted with the area size of electrodes incontact with the piezoelectric layer 23 to form resonators whereresonance occurs.

1. An electric filter comprising a plurality of thin film bulk acousticresonators (FBARs) consisting of a thin layer of a piezoelectricmaterial sandwiched between two metal electrodes linked in aseries/parallel connection arrangement for which the areas of theelectrodes in contact with the piezoelectric layer to form theresonators are different between in series and in parallel FBARs.
 2. Anelectric filter as described in claim 1, wherein the thickness of thepiezoelectric material is different between the in series and inparallel FBARs.
 3. An electric filter as described in claim 1 or claim2, wherein the areas of the electrodes for the FBARs linked in seriesare adjusted so that their series resonance frequency is the same as theparallel resonance frequency of the FBARs linked in parallel.
 4. Anelectric filter as described in any preceding claim, wherein thepiezoelectric material is zinc oxide.
 5. An electric filter as describedin any of claims 1 to 3, wherein the piezoelectric material issubstantially comprised of lead titanate zirconate.
 6. An electricfilter as described in any of claims 1 to 3, wherein the piezoelectricmaterial is aluminum nitride.
 7. An electric filter as described in anyof claims 1 to 3, wherein the piezoelectric material is substantiallycomprised of lead scandium tantalum oxide.
 8. An electric filter asdescribed in any of claims 1 to 3, wherein the piezoelectric material issubstantially comprised of bismuth sodium titanium oxide.
 9. An electricfilter as described in any preceding claim, wherein the metal electrodescomprise gold.
 10. An electric filter as described in any of claims 1 to8, wherein the metal electrodes comprise aluminum.
 11. An electricfilter as described in any of claims 1 to 8, wherein the metalelectrodes comprise platinum.
 12. An electric filter as described in anypreceding claim, wherein 2 FBARs are linked in series and 2 FBARs arelinked in parallel.
 13. An electric filter as described in any one ofclaims 1 to 11, whereon 3 FBARs are linked in series and 3 FBARs arelinked in parallel.
 14. An electric filter according to any precedingclaim, wherein the piezoelectric material of FBARs in parallel isthicker than that of FBARs in series.
 15. An electric filter accordingto any preceding claim, wherein the area of electrodes of FBARs inparallel is greater than that of FBARs in series.
 16. An electric filtercomprising at least one FBAR in series and at least one FBAR inparallel, each FBAR comprising a layer of piezoelectric materialsandwiched between two electrodes of which the areas of the electrodesin contact with the piezoelectric layer are different between the FBARin series and the FBAR in parallel.
 17. An electric filter substantiallyas hereinbefore described with reference to the accompanying drawings.